Respiratory distress syndrome, which mostly affects premature newborns and can lead to ongoing breathing problems, blindness and brain damage, was more common in the babies born by cesarean, or C-section, delivery, researchers found.

"I would say that we at least showed that there may not be any benefit to cesarean delivery in preterm births, and more research is needed before the C-section rate goes up even further," said study leader Dr. Erika Werner, an assistant professor of maternal-fetal medicine at the Johns Hopkins School of Medicine in Baltimore.

Between 2002 and 2009, cesarean deliveries in the United States rose from 27 percent of births to 34 percent, according to a HealthGrades report. The U.S. Centers for Disease Control and Prevention recently reported a dip in rates to slightly under 33 percent in 2010.

Groups such as the American Congress of Obstetricians and Gynecologists are trying to curb cesarean deliveries, but without much success.

"I'm acutely aware as a clinician that the C-section rate is rising, and there were prior studies that really had mixed results," Werner said. "Some suggested an increased risk of bleeding in the brain and some suggested increased risk of death in this population with a vaginal delivery. So I really wanted to look at a contemporary, diverse population."

The researchers analyzed birth data on 2,560 small-for-gestational-age babies delivered between 25 and 34 weeks of pregnancy in New York City hospitals from 1995 to 2003. (Before 37 weeks is considered preterm). Forty-six percent were delivered vaginally and 54 percent by C-section. The study only looked at live births and excluded mothers who had a previous cesarean delivery.

Vaginal deliveries that used forceps or vacuum were excluded from the study, but they're not usually a factor with such small newborns, Werner explained.

The study found no difference in the incidence of brain bleeds, seizure or sepsis between the two types of delivery. While cesarean-delivered babies had lower Apgar scores — a measure of newborn health — the difference was not significant after taking other factors into account. But the higher risk of respiratory distress syndrome still held.

The study is slated for presentation Thursday at the annual meeting of the Society for Maternal-Fetal Medicine in Dallas.

Dr. Diane Ashton, deputy medical director of the March of Dimes, who was not involved in the new study, said the results echo other recent findings.

"This particular study shows that for infants that are premature and small-for-gestational-age, which tend to be thought of as rather fragile infants, the cesarean delivery doesn't offer any protection from the adverse outcomes," Ashton said.

In general, the March of Dimes stance "is that elective deliveries — and that's cesarean sections and inductions — should not occur for infants born before 39 weeks unless there's a medical indication," she noted.

"For those instances where there is a strong medical indication, either because of the mother's health or the fetus' health, I think it's important that women understand that it's a major surgery and that it can be associated with complications," Ashton added.

Werner said her team did a similar study on premature infants whose weight fell in the normal range. "We had very similar outcomes, although the respiratory distress was even more impressive and there was more of a benefit to vaginal delivery," she said.

Ashton said that vaginal birth helps babies breathe. "The process of delivering babies vaginally and going through the birth canal and getting the amniotic fluid out of the lungs can enhance their respiratory status," she explained.

While the new study found an association between cesarean deliveries and breathing problems in preterm babies, it did not prove a cause-and-effect relationship.

The study relied on birth certificate and hospital-discharge data. "We don't know if C-section just selects a population more at risk for respiratory distress or if C-section causes respiratory distress. We can't tell which comes first in this kind of study," Werner said.

Ashton said it's not too soon for these and similar findings to influence practice.

"Certainly there should be some review and maybe recommendations from the American Congress of Obstetricians and Gynecologists as to where they think practice should change," Ashton said.

Data and conclusions presented at medical meetings should be viewed as preliminary until published in a peer-reviewed medical journal.

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